1. Field of the Invention
The present invention relates to a curl correcting device for correcting-the curl of a sheet.
2. Description of the Related Art
In general, an image forming apparatus using an electrophotographic technique forms an image by developing an electrostatic latent image formed on an image carrier (such as a photosensitive member) in accordance with image information by a developer in order to form a toner image. Then, the apparatus transfers the toner image onto a sheet either directly or indirectly through an intermediate transferring unit. Then, the apparatus passes the sheet between a fixing heating roller and a pressure roller or a belt of a press-contact section in order to fix the toner image to the sheet. After the image is fixed to the sheet, the sheet may curl in the direction of the front or the back of the sheet. Therefore, there are some image forming apparatuses provided with a curl correcting device for correcting a sheet curl that is produced after the fixing operation in order to prevent problems caused by the curling, such as jamming of sheets, an increase in the space taken up by sheets at a sheet-discharge unit, and a reduction in the quality of images formed on both sides of a sheet.
The following types of curl correcting devices are primarily known. They are a roller nip type and a belt nip type. The roller nip type comprises an elastic roller and a pressure roller which rotates at a curved nip transporting section that is formed when the pressure roller presses the elastic roller. The belt nip type comprises a belt and a pressure roller. The belt rotates by being disposed on a belt supporting roller. The pressure roller rotates at a curved nip transporting section that is formed when the pressure roller presses the belt supporting roller. These curl correcting devices correct a sheet curl by passing the sheet to which an image is fixed through the curved nip transporting section (formed by the elastic roller or the belt and the pressure roller) in order to temporarily curve the curled sheet in a direction opposite to the direction of curl of the sheet.
Since the nature of the curl occurring after the fixing depends upon, for example, the type of sheet, the density of an image formed on a sheet, and the humidity of the environment, such curl correcting devices adjust the curl correction amount in accordance with a predetermined parameter, such as the type of sheet or the image density. In other words, in general, such curl correcting devices adjust the curl correction amount by changing the pressing amount of the pressure roller against the elastic roller or the belt, so that the curl correction amount (that is, the pressing amount of the pressure roller) is changed and controlled in accordance with detection information regarding, for example, the type of sheet used, the image density, or the humidity.
In general, such curl correcting devices adjust the curl correction amount by previously providing and storing control information in which a condition, such as the sheet type, the image density, or the humidity, is associated with the curl correction amounts, and by automatically controlling the pressing amount of the pressure roller on the basis of the control information.
In recent years, in the print on demand (POD) market, a post-processor which can be used to load a large number of sheets, such as a stacker, is becoming important. The required curl correction amount when sheets are discharged onto a sheet-discharge tray from the body of the image forming apparatus and that when the sheets are loaded onto a post-processor, such as a stacker, from the body of the image forming apparatus are different due to loading conditions, such as a difference in height from a sheet-discharge opening to a loading surface. In general, in a stacker for loading a large number of sheets, the distance from the sheet-discharge opening to the loading surface is large. Therefore, a sheet that is curled downward tends to buckle when it lands in the stacker. In order to prevent this, it is necessary to eliminate the downward curl compared to the case where the sheet is discharged onto the sheet-discharge tray. Accordingly, it is necessary to limit the curl correction strength applied to the sheet in accordance with the connected post-processor, so that a controlling operation for limiting the curl correction strength is carried out.
Due to the diversification of sheets in recent years, a sheet curl may not be properly corrected by the above-described automatic correction. One example of such a sheet is a sheet having a surface coated with resin and being frequently used in the POD market. Such a coated sheet tends to be curled by toner because it has low resilience compared to a related non-coated sheet of the same thickness (basis weight). Therefore, it is not enough to merely classify sheets in accordance with the thickness as a parameter as is conventionally the case. In addition, the amount of curl of a coated sheet depends on, for example, the amount and the composition of the resin used for coating, and whether or not one side or both sides of the sheet are coated. Therefore, it is difficult to carry out the same controlling operation on the correction amount of different types of coated sheets.
Even if a sheet is of a type whose curl correction amount can be automatically adjusted, the curl of the sheet type may not be properly corrected depending upon, for example, the humidity of the environment and the state in which the sheet type is left standing.
In order to overcome this problem, Japanese Patent Laid-Open No. 2002-80157 discloses a curl correcting device comprising means for manually correcting a sheet curl in order to make it possible to properly correct the sheet curl.
However, when a post-processor is mounted, and the curl correction strength is limited, proper correction cannot be carried out due to the diversification of sheets.